Patent Literature 1 discloses that, by decreasing the binding phase (iron group metal) concentration at the surface portion of a nitrogen-containing TiC-based cermet to be lower than that in the interior portion so that the percentage of the hard phase at the surface portion is increased, a compressive residual stress of 30 kgf/mm2 or more is made to remain at the surface portion of the sintered compact, thereby improving wear resistance, fracture resistance, and thermal shock resistance.
In addition, Patent Literature 2 also discloses that, when WC grains constituting the main crystal phase of a WC-based cemented carbide have a compressive residual stress of 120 kgf/mm2 or more, the WC-based cemented carbide has high strength and good fracture resistance.
Furthermore, Patent Literature 3 describes that, when residual stresses of different magnitudes are applied to a first hard phase and a second hard phase, it is possible to suppress occurrence or development of cracks in the cermet, resulting in improvement in the chipping resistance of the cutting tool.
The following is a list of aforementioned background art:    Patent Literature 1: Japanese Unexamined Patent Application Publication No. H05-9646    Patent Literature 2: Japanese Unexamined Patent Application Publication No. H06-17182    Patent Literature 3: International Publication Pamphlet No. 2010/013735
However, in the method in which residual stress is generated in a sintered cermet body by setting the surface portion and the interior portion to have different contents of the binding phase as in Japanese Patent Application Publication No. H05-9646 described above, since the content ratio of the binding phase with respect to the entire cermet is low, a sufficient amount of residual stress is not applied to the entire cermet, resulting in difficulty in obtaining satisfactory toughness.
Furthermore, in the method in which residual stress is applied uniformly to the hard phase as in Japanese Patent Application Publication No. H06-17182, there is also a limitation in improving the strength of the hard phase.
Furthermore, in the structure of International Publication Pamphlet No. 2010/013735 described above, although the chipping resistance of the cermet as a cutting tool is improved, the cermet tends to have a large deformation due to firing compared to cemented carbides, and when a cutting tool having a complex shape with a plurality of corner cutting edges is produced, there may be a difference in the location or shape of the corner cutting edge depending on the corner used, resulting in problems, such as differences in size when cutting is performed.
The present invention has been achieved to solve the problems described above. It is an object of the present invention to provide a cutting tool having high working accuracy by improving the toughness of a cermet and reducing deformation due to firing.